The present invention relates to optical scanners and more specifically to a dynamic laser diode aperture for optical scanners.
Optical scanners are well known for their usefulness in retail check out and inventory control. Optical scanners generally employ a laser diode, the light from which is focused and collimated to produce a scanning beam. An optical transceiver directs the beam against a plurality of stationary mirrors, and collects the beam after it is reflected by a bar code label. A motor rotates the optical transceiver, and a detector receives the returning beam. The pattern produced by such a scanner is characterized by lines oriented at various angles to one another. An example of an optical scanner can be found in commonly assigned U.S. Pat. No. 4,971,410, entitled, "Scanning and Collection System for a Compact Laser", issued Nov. 20, 1990, to Wike, Jr. et al. This patent is hereby incorporated by reference.
The laser diodes used in optical scanners exhibit both desirable and undesirable characteristics. Unfocused laser light produces a scanning ellipse, which is suitable only for use in optical scanners which generate scan lines in one direction, and which is suitable for scanning bar code labels as long as the minor axis of the ellipse is oriented perpendicular to the lines and spaces of the bar code label. Furthermore, laser output power has two propagation axes, one axis diverging around thirty-five degrees and the other axis diverging around ten degrees. Thus, the asymmetry of the scanning ellipse makes light collection techniques in optical scanners difficult. On the other hand, light from the scanning ellipse generates more optical power than focused and collimated light, and reduces print defects by averaging them to yield a modulated signal with less print-generated noise.
Known focusing techniques for optical scanners, such as the scanner disclosed in Wike, Jr., employ a single focusing lens followed by a circular aperture. The circular aperture determines the size of the laser beam projected at a reference plane. Since the size of the aperture remains constant, it may be referred to as a "static" aperture. However, the circular static aperture attenuates as much as eighty-five percent of the available light power from the laser diode because the diameter of the circular aperture must be limited to maintain the optimum "F" number for a proper beam profile about the primary focus.
Therefore, it would be desirable to produce an optical scanner which incorporates the high power and low noise advantages of the scanning ellipse.